Point mutations in connexin proteins can cause gain in hemichannel function (e.g., exacerbated hemichannel opening), which results in human pathologies, including deafness, skin disorders, cataract and Charcot-Marie-Tooth disease. Due to the large size and modest selectivity of the aqueous pore, exacerbated opening of connexin hemichannels at the plasma membrane leads to loss of electrochemical gradients and of small cytoplasmic metabolites, causing cell death. Control of hemichannel opening is indispensable, and is achieved by physiological extracellular Ca2+, which drastically reduces hemichannel activity. Aberrantly open hemichannels caused by connexin mutations are less sensitive to extracellular Ca2+. Some of these mutations are located at the intracellular site end of the pore, suggesting that they affect gating (opening and closing of the pore) rather than the Ca2+ binding site itself. To explain the relationship between these mutations and Ca2+ regulation in hemichannels, we hypothesize that Ca2+ binds to and stabilizes the closed hemichannel. Mutations that produce gain of function decrease occupancy of the closed state, rendering the channel less sensitive to Ca2+. The goal of this project is to identify the molecular basis of regulation of connexin hemichannels by external Ca2+ and the mechanistic basis of hemichannel gain of function induced by mutations in human connexin26 (hCx26) that cause disease. The crystal structure of the hCx26 channel was recently solved and will serve as a guide for structure-function studies. We hope that a better understanding of the mechanisms of gating and Ca2+ regulation of connexin channels will lead to development of drugs and other therapeutic approaches that can specifically correct or compensate for hemichannel gain of function, and hopefully serve, in the case of hCx26, to treat deafness and skin disorders caused by mutation of this connexin.